Implantable cardiac pacemakers restore normal heart rhythm by delivering external electrical pacing to the heart. The pacemaker software is life-critical as the timing of the pulses determine its ability to control the heart rate. Recalls due to software issues have been on the rise with the increasing complexity of pacing algorithms. Open-loop testing remains the primary approach to evaluate the safety of pacemaker software. While this tests how the pacemaker responds to stimulus, it cannot reveal pacemaker malfunctions which drive the heart into an unsafe state over multiple cycles. To evaluate the safety and efficacy of pacemaker software we have developed a heart model to generate different heart conditions and interact with real pacemakers. In this paper, we introduce the closed-loop testing platform which consists of a programmable hardware implementation of the heart that can interact with a commercial pacemaker in closed-loop. The heart-on-a-chip implementation is automatically generated from the Virtual Heart Model in Simulink which models different heart conditions. We describe a case study of Endless Loop Tachycardia to demonstrate potential closed-loop pacemaker malfunctions which inappropriately increase the heart rate. The test platform is part of our model-based design framework for verification and testing of medical devices with the patient--in-the-loop.